1. Field of the Invention
The present invention relates to a lash-eliminating device for a worm-and-gear assembly, and in particular to an electronic device with a worm-and-gear assembly that can be accurately positioned at a low cost.
2. Description of the Related Art
In a feed mechanism of an electronic device, such as an optical disc drive, gears are mainly used as a transmission unit. To avoid interference between adjacent gears, a lash is formed between adjacent gears. However, the lash becomes a non-linear factor between a rotation of an output shaft of a motor of the optical disc drive and a movement of an optical pickup of the optical disc drive. Thus, the optical pickup is difficult to position accurately. Furthermore, when the optical disc drive is subjected to vibration, the lash becomes a source of free vibration so as to affect the position of the optical pickup. An additional biasing member is provided to reduce the lash between adjacent gears.
In an electronic device with low-noise requirements, such as an optical disc drive with high speed, a worm-and gear assembly is usually used as a feed mechanism since it is provided with low noise and high contact ratio. FIG. 1a and FIG. 1b show a conventional optical disc drive with a worm-and-gear assembly as a feed mechanism. In FIG. 1a and FIG. 1b, reference 11 represents a motor, reference 12 a worm, and reference 13 a worm gear. To simplify the figures, teeth of the worm 12 and the worm gear 13 are omitted. The shape of the worm-and-gear assembly is very complicated. Furthermore, when the worm-and-gear assembly is operated, it generates force in three directions; axial, radial, and tangential. In FIG. 1a, an arrow al represents the axial direction. In FIG. 1a and FIG. 1b, an arrow a2 represents the radial direction. In FIG. 1b, an arrow a3 represents the tangential direction. It is noted that the directions of the force in the axial direction al and the tangential direction a3 depend on the direction of the rotation of the worm 12. Thus, if the direction of the biasing force of the biasing member is the same as that of the operational worm, the worm and the worm gear jam. As a result, the difficulty of the design of the biasing member is increased.
A lash-eliminating device for a worm-and-gear assembly is often applied to a dividing plate. Referring to FIG. 2, the dividing plate includes two worms 25, 26. Power from a motor 21 is transmitted to a worm gear 24 via a one-way clutch 22 and a torsional restrainer 23. When the power is transmitted via one worm 25, the other worm 26 generates a force in an opposite direction so as to abut the worm gear 24 to eliminate the lash. However, since such mechanism includes two worms and a complicated switch unit, the cost is high. Furthermore, it is difficult to apply to electronic devices with limited space and weight, such as the optical disc drive.
It is noted that the invention is designed for electronic devices such as CD-R/RW, CDR/RW-DVD, and DVD-RW, requiring limited space and weight, and higher positional accuracy. Since the feed mechanism including spur gears has difficulty attaining high reduction ratio in a limited space, it is not applied thereto. In addition, in a conventional feed mechanism, a lashless leading screw is used. However, the cost of such feed mechanism is higher then other feed mechanism.